Rangefinder and method for collecting calibration data

ABSTRACT

An apparatus and method for calibrating range measurements are provided wherein calibration data is collected with each range measurement or group of range measurements. The calibration data comprise a plurality of simulated range measurements. In one embodiment, the simulated range measurements are used to analyze errors that vary with time and environmental conditions. Range measurements are calibrated by correlating a measured flight time of a transmitted and reflected laser beam with the simulated range measurements and a relationship between laser beam flight times and target ranges based on the speed of the laser beam.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 10/793,144, filed Mar. 4, 2004, entitled “Rangefinder andMethod for Collecting Calibration Data, ” and claims priority benefitunder 35 U.S.C. § 120 to the same. Moreover, the present applicationclaims priority benefit under 35 U.S.C. § 119(e) from U.S. ProvisionalApplication No. 60/525,621, filed Nov. 26, 2003, entitled “Rangefinderand Method for Collecting Calibration Data. ” The present applicationincorporates the foregoing disclosures herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rangefinder device for measuring adistance to a target.

2. Description of the Related Art

Rangefinders are used to measure distances to targets. Typically, arangefinder will emit a beam of energy towards a target and detect areflected beam from the target. The rangefinder measures the timeinterval between the emission of the transmitted beam and the receptionof the reflected beam. This time interval is referred to herein as the“flight time.” The distance from the rangefinder to the target isderived from the speed of the beam and the flight time.

The accuracy of range measurements is affected by the rangefinder'sability to accurately measure a beam's flight time because smallvariations can create significant errors in the distance calculated.Often, capacitor discharge mechanisms are used to create a moremanageable representation of the flight time. Even using suchdischarging mechanisms, delays in the rangefinder's internal circuitryadd additional error to flight time interval measurements.

Some errors caused by the internal circuitry are due to inherent delaysthat can be initially measured and corrected during, for example,factory calibration and test. However, some delays caused by theinternal circuitry are variable and may change over time. Further, somedelays may change with variations in environmental conditions such astemperature, humidity and the like.

SUMMARY OF THE INVENTION

Thus, it would be advantageous to develop a technique and system forcalibrating range measurements using data collected each time arangefinder acquires a range measurement or a set of range measurements.It would also be advantageous to develop a technique and system tocorrect range measurement errors related to dynamic factors, such aserrors that vary with time or environmental conditions.

The present invention provides a rangefinder and method for calibratinga target range measurement. A rangefinder according to the inventionperforms a calibration each time a range measurement is taken. Therangefinder collects an uncalibrated range measurement by measuring theflight time of an energy or light beam as it travels to and from atarget. The rangefinder automatically generates calibration data bysimulating range measurements. The rangefinder uses the calibration datato correct measurement errors and outputs a calibrated rangemeasurement.

According to the foregoing, an embodiment includes a method forcalibrating a rangefinder. The method includes determining a firstrelationship between flight times and target ranges. The method alsoincludes generating a first simulated range measurement by measuring afirst discharge time of a capacitor, such as a timing capacitor. Asecond simulated range measurement is generated by measuring a seconddischarge time of the capacitor. The first and second simulated rangemeasurements are used to calculate a second relationship between flighttimes and target ranges. The second relationship is used to correlate anuncalibrated range measurement to the first relationship and a distanceto a target is determined.

In an embodiment, a rangefinder is configured to determine a calibratedrange to a target. The rangefinder comprises a transmitter configured toemit a beam towards the target, a receiver configured to detect areflected beam from the target, and timing circuitry configured tomeasure a flight time between the emission of the beam from thetransmitter and a detection of the reflected beam by the receiver. Therangefinder also includes a calibration section configured to determinecalibration data related to dynamic factors, such as errors that varywith time or environmental conditions. The rangefinder further includesa processor configured to adjust the flight time based on thecalibration data.

In an embodiment, a system is provided for measuring a range to atarget. The system includes a means for storing a first parameterproportional to a flight time of a beam and for storing second and thirdparameters proportional to respective first and second calibrationtimes. The system also includes a means for measuring the stored first,second and third parameters to respectively produce an uncalibratedrange measurement and first and second simulated range measurements, anda means for correlating the uncalibrated range measurement to the firstand second simulated range measurements.

In an embodiment, a method is provided for measuring a range to a targetwherein a first time corresponding to a beam traveling between the rangefinder and the target is measured and calibration data is collected bysimulating a second time and a third time corresponding to the beamtraveling between the range finder and the target. The method alsoincludes correlating the first time to the second time and the thirdtime and outputting a range.

Other features and advantages of the present invention will becomeapparent to those of ordinary skill in the art through consideration ofthe ensuing description, the accompanying drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

A system and method which embodies the various features of the inventionwill now be described with reference to the following drawings:

FIG. 1 is a perspective view of an exemplary laser rangefinder accordingto an embodiment of the invention;

FIG. 2 is an exemplary block diagram illustrating a range determinationprocess usable by the rangefinder of FIG. 1;

FIG. 3 is an exemplary illustration of a display of the rangefinder ofFIG. 1;

FIG. 4 is a block diagram illustrating a rangefinder system according toan embodiment of the invention;

FIG. 5 is a flowchart of an exemplary data collection process usable bythe rangefinder system of FIG. 4;

FIG. 6 is a simplified schematic of an exemplary timing circuitaccording to an embodiment of the invention;

FIGS. 7A and 7B are exemplary graphical representations illustrating atarget range versus a flight time of a laser beam;

FIGS. 8A and 8B are exemplary graphical representations illustratingcharge in a capacitor during first and second calibration measurements;and

FIGS. 9A and 9B are exemplary graphical representations illustrating atarget range versus a flight time of a laser beam, an uncalibrated rangemeasurement, a first simulated range measurement, and a second simulatedrange measurement.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention involves a rangefinder system which performs acalibration each time a range measurement is taken. In an embodiment ofthe rangefinder system, a raw or uncalibrated range measurement iscollected when a user triggers the rangefinder. Alternatively, multipleuncalibrated range measurements are collected when the user triggers therangefinder. Collecting an uncalibrated range measurement involvesmeasuring the flight time of a beam as it is transmitted to a target andreflected back to the rangefinder system.

Once the uncalibrated range measurement is collected, the rangefindersystem of the present invention automatically generates calibrationdata. Alternatively, the calibration data is generated when the usertriggers the rangefinder, before the uncalibrated range measurements arecollected. Preferably, the calibration data is generated in relation tothe time that the uncalibrated range measurement is collected so as todetermine measurement errors related to dynamic factors, such as errorsthat vary with time or environmental conditions.

According to one aspect of the rangefinder system, calibration data isgenerated by simulating range measurements. During a simulated rangemeasurement, the rangefinder system measures a known flight time andcalculates a measurement error based on the difference between themeasured flight time and the known flight time. The rangefinder systemcalibrates the uncalibrated range measurement by correcting for thecalculated measurement error. The rangefinder system then provides thecalibrated range measurement to the user.

In the following description, reference is made to the accompanyingdrawings, which form a part hereof, and which show, by way ofillustration, specific embodiments or processes in which the inventionmay be practiced. Where possible, the same reference numbers are usedthroughout the drawings to refer to the same or like components. In someinstances, numerous specific details are set forth in order to provide athorough understanding of the present invention. The present invention,however, may be practiced without the specific details or with certainalternative equivalent components and methods to those described herein.In other instances, well-known components and methods have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

FIG. 1 is a perspective view of an exemplary laser rangefinder 100according to an embodiment of the invention. The laser rangefinder 100comprises a housing 112, user optics 114, laser optics 116, a display118, a power or trigger actuator 120 and a mode selector 122. The laserrangefinder is portable and is configured to be held in a user's handwhile taking range measurements. For example, the rangefinder 100 may beused in nature watching, such as bird watching, sports such as golf,hunting, or the like.

Although described with reference to a handheld monocular rangefinder,an artisan will recognize from the disclosure herein that therangefinder 100 ma be a binocular device, camera, gun, other opticaldevice, or the like. The laser rangefinder 100 may be mounted on amoveable or fixed surface or stand such as a camera tripod or the like.

FIG. 2 is an exemplary block diagram illustrating a range determinationprocess usable by the rangefinder of FIG. 1. Referring to FIGS. 1 and 2,the display 118 and user optics 114 are used to align the laser optics116 with a remote target 224. Pressing the trigger actuator 120 causesthe laser rangefinder to emit a laser beam 220 toward the remote target224 through the laser optics 116. In an embodiment, the transmittedlaser beam 220 can comprise a pulsed laser beam. The laser rangefinderis configured to detect a reflected laser beam 222 from the target 224through the laser optics 116. The laser rangefinder is configured tomeasure the flight time of the transmitted laser beam 220 and thereflected laser beam 222, and to calculate a range from the laserrangefinder 100 to the target 224. In an embodiment, the range is basedon the measured flight time (often divided by two) and the speed of thetransmitted and reflected laser beams 220, 222. For example, taking thespeed of the transmitted and reflected laser beams to be about 0.98357feet per nanosecond, the range to the target may be calculated inequation (1) as: $\begin{matrix}\begin{matrix}{{Range} = {0.98357\quad\left( {{feet}\text{/}{nanosecond}} \right) \times \left( {{flight}\quad{time}\quad{({nanoseconds})/2}} \right)}} \\{{= {0.49178\quad\left( {{feet}\text{/}{nanosecond}} \right) \times {flight}\quad{time}\quad({nanoseconds})}},}\end{matrix} & (1)\end{matrix}$where 0.98357 feet per nanosecond represents the speed of light in avacuum and is provided for exemplary reasons only and is not intended tolimit or construe the disclosure or claims. In fact, an artisan willrecognize from the disclosure herein many possible light or energy beamspeeds that can be used.

As discussed in detail hereinbelow, an embodiment of the laserrangefinder is configured to calibrate a raw or uncalibrated rangemeasurement derived from equation (1) to account for errors introducedby internal circuitry delays, aging, and environmental changes.

The display 118 comprises a monocular eyepiece coupled to the useroptics 114. Alternatively, the display 118 can comprise binoculareyepieces wherein each eyepiece is coupled to separate user optics (notshown) or to the same user optics 114. As another alternative, thedisplay 118 may comprise a video display such as a liquid crystaldisplay (LCD) screen or the like. Additionally, or alternatively, anartisan will recognize from the disclosure herein a variety oftechniques for allowing a user to effectively aim the rangefinder 100 atone or more potential remote targets.

FIG. 3 is an exemplary illustration of a user display 300 of therangefinder of FIG. 1. The user display 300 is visible, for example,when looking at or through the display 118 shown in FIG. 1. The userdisplay 300 can comprise targeting indicia 310 configured to aid a userwhen aligning the laser optics 116 with a remote target (not shown).Thus, in one embodiment, the user display 300 comprises a transparentbackground which allows the user to see both the target and thetargeting indicia 310

The user display 300 also comprises range and speed indicia 312 andcorresponding active unit indicia 314 configured to display the distanceto or speed of a remote target. A user may select the units in which todisplay a measurement and the corresponding units will be displayed inthe active unit indicia 314. For example, speed may be displayed askilometers/hour, miles/hour, or the like and distance may be displayedas feet/second, meters/second, yards/second, or the like. The user mayselect the units in which to display the range/speed indicia 312 by, forexample, pressing the mode selector 122 shown in FIG. 1 a predeterminednumber of times or for a predetermined length of time.

The user display 300 may also comprise a power indicator 316 and modeindicators 318. In one embodiment, the power indicator 318 is displayedwhen a low battery condition exists. The mode indicators 316 display thecurrent mode of the laser rangefinder which may be selected by pressingthe mode selector 122 shown in FIG. 1. For example, “RAIN” is displayedwhen rain mode is selected to remove the effects of rain, snow andflying insects from the range measurement, “>150” is displayed when longrange mode is selected to suppress reflections from objects such asbushes and trees that are between the rangefinder and a distant target(e.g., a target greater than 150 yards away), “SPD” is displayed whenspeed mode is selected to measure the speed of a target, and “CONF” isdisplayed when configuration mode is selected to configure therangefinder (e.g., to configure the displayed units). In one embodiment,the mode indicators 318 comprise a reflection signal strength indicator“REFL” which is displayed when a strong target reflection signal isdetected. Alternatively, the reflection signal strength indicator REFLmay comprise a gauge or a bar graph which indicates the relativestrength of the detected target reflection signal.

Although discussed with reference to one or more embodiments visiblethrough the user optics of the rangefinder 100, an artisan willrecognize from the disclosure herein a number of alternatives for theuser display 300 of FIG. 3. For example, the user display 300 maycomprise an attached or detached viewable display, such as thoseassociated with, for example, camcorders, laptops, cell phones, personaldigital assistants (PDAs), other computing devices, or the like. Also,the rangefinder may include communication mechanisms, such as a signaloutput, that communicates with one or more of the foregoing devices in awired or wireless manner. Thus, the rangefinder 100 can be configured totransmit range data to an external device or system for furtherprocessing or display. For example, the rangefinder 100 may beconfigured to transmit range information to a system configured toadjust the location of targeting indicia, such as cross-hairs or thelike, in a scope or other siting device based on the range information.In an alternative embodiment, the rangefinder 100 is configured toprovide an audible indication of range information through, for example,a loudspeaker, headphones, or the like.

FIG. 4 is a block diagram illustrating a rangefinder system 400according to an embodiment of the invention. The rangefinder system 400comprises a controller 410 coupled to a transmitter 412 and a receiver414 through timing circuitry 440. The transmitter 412 is configured toemit a laser beam and the receiver 414 is configured to detect areflection of the emitted laser beam. In an embodiment, the transmitter412 and the receiver 414 can be coupled to a high voltage power supply416.

The controller 410 comprises, by way of example, one or more processors,program logic, hardware, software, or other substrate configurationscapable of representing data and instructions which operate as describedherein or similar thereto. The controller 410 may also comprisecontroller circuitry, processor circuitry, processors, general purposesingle-chip or multi-chip microprocessors, digital signal processors,embedded microprocessors, microcontrollers, combinations of theforegoing, or the like. The controller 410 further comprises a counter418. In an alternative embodiment, the counter 418 is external to thecontroller 410.

In one embodiment, the controller 410 also includes an internal memorydevice 420 comprising, for example, random access memory (RAM). Thecontroller can also be coupled to an external memory device 424comprising, for example, drives that accept hard and floppy disks, tapecassettes, CD-ROM or DVD-ROM. The internal memory device 420 or theexternal memory device 424 or both, can comprise program instructions422, 426 for controlling the timing circuitry 440, transmitting andreceiving laser beams, storing data including range measurements andcalibration data, performing statistical analysis on the measured data,and calibrating measured data as described herein.

The controller 410 is coupled to a display 428 a communication device436, a user input device 432, and a power supply 430. In one embodiment,the display 428 is an LCD screen attached to the rangefinder system 400configured to display a target and, for example, some or all a portionof the indicia and indicators discussed above with respect to FIG. 3. Asdisclosed in the foregoing, other embodiments of the display 428include, for example, an optical viewfinder for locating a target and aseparate digital display for indicating a range or speed to the target,a detachable video monitor such as a cathode ray tube (CRT), an LCDsuperimposed on an optical viewfinder, or the like. The communicationdevice 436 is configured to provide communication with external systemsand devices and can comprise, for example, a serial port, a parallelport, a universal serial bus (USB) controller, or an Internet or othernetwork adapter. The user input device 432 can include, for example, akeypad, a mouse, user buttons such as the trigger actuator 120, the modeselector 122 shown in FIG. 1, or any device that allows a user to enterdata into the controller 410.

According to an embodiment, the timing circuitry 440 comprises a timegeneration section 442, a calibration section 444 and a time measurementsection 446. The time generation section 442 is configured to correlatethe start of a timing parameter (not shown) with the transmission of alaser pulse by the transmitter 412. The time generation section 442 isconfigured to initiate the timing parameter and to command thetransmitter 412 to emit a laser pulse in response to a transmit signal(not shown) received from the controller 410. The timing parameter cancomprise, for example, a physical parameter stored as a function oftime, such as a charge stored in a capacitor, or the like.Alternatively, the timing parameter can comprise, for example, a timevalue generated or stored by a counter, a timer, or the like.

The calibration section 444 is configured to remove errors inuncalibrated range measurements due to internal circuitry delays, aging,and environmental conditions such as temperature, humidity and the like.In an embodiment, the calibration section 444 is configured to simulateone or more range measurements by ignoring the reflected laser pulse andstopping the timing parameter at predetermined calibration times. Thecalibration section 444 corrects range measurement errors by correlatinguncalibrated range measurements with the one or more simulated rangemeasurements to create a calibrated range. In an embodiment, thecalibration section 444 simulates one or more range measurements eachtime an uncalibrated range measurement is collected. Alternatively, thecalibration section 444 simulates one or more range measurements eachtime a set of uncalibrated range measurements is collected.

The time measurement section 446 is configured to correlate the timingparameter with a flight time of a transmitted and reflected laser pulse.In an embodiment, the time measurement section 446 is configured to stopthe timing parameter in response to a reflected laser pulse detected bythe receiver 414 and to measure the timing parameter. The measuredtiming parameter corresponds to a flight time measurement betweentransmission of the laser pulse by the transmitter 412 and detection ofthe laser pulse by the receiver 414. In an embodiment, the timemeasurement section 446 is configured to stop the timing parameter at apredetermined calibration time and to measure a timing parametercorresponding to a simulated range measurement.

FIG. 5 illustrates an exemplary data collection process 500 usable by arangefinder, such as the rangefinder system 400 of FIG. 4. Thecollection process 500 comprises, in short, collecting rangemeasurements, generating calibration data, calibrating, and outputting acalibrated range. Thus, calibration data is generated each time rangemeasurements are collected. This allows the calibration data to accountfor range measurement errors that vary with time or environmentalconditions.

Referring to FIG. 5 at block 502, a rangefinder 400 collects rangemeasurements. In an embodiment, range measurements are collected bymeasuring the flight time of transmitted and reflected energy or lightbeams. The measured flight time and speed of the beam is used tocalculate a distance from the rangefinder 400 to a target.

At block 504, the rangefinder 400 generates calibration data. Thecalibration data is proportional to an error in the collected rangemeasurements. The error may, for example, be related to dynamic factorsor inherent delays in the circuitry of the rangefinder 400. In anembodiment, the calibration data is automatically generated after therangefinder 400 collects a range measurement. Alternatively, thecalibration data is automatically generated before the rangefinder 400collects a range measurement. Preferably, the calibration data isgenerated within a period of time before or after the range measurementis collected so as to provide a measurement of errors related to dynamicfactors, such as errors that vary with time or environmental conditions.

In an embodiment, the rangefinder 400 generates calibration data bysimulating one or more range measurements. Range measurements may besimulated, for example, by measuring a predetermined calibration timeand comparing the measured flight time with the predeterminedcalibration time to determine an error. In an embodiment, two or morerange measurements are simulated to determine an error relationshipbetween flight times and target ranges.

At block 506, the rangefinder 400 calibrates the collected rangemeasurements by correcting for the error proportional to the calibrationdata. At block 508, the rangefinder 400 outputs the calibrated range.The calibrated range may be output, for example, by communicating thecalibrated range value to a display device, an external memory device, acommunication device, or the like.

FIG. 6 is a simplified schematic of an exemplary timing circuit 440according to an embodiment of the invention. In the illustratedexemplary embodiment, the timing circuitry 440 comprises three switches610, 612, 614 coupled to a capacitor 616 at an input of a comparator618. The capacitor 616 is coupled between the “-” input terminal of thecomparator 618 and circuit ground 620. The “+” input terminal of thecomparator 618 is coupled to circuit ground 620.

Switch 610 is configured to switch the capacitor 616 to a chargingsignal +VCC through a current source 622 in response to a “Ramp Up”signal. Switch 612 is configured to switch the capacitor 616 to circuitground 620 in response to a “Reset Ramp” signal. Switch 614 isconfigured to switch the capacitor 616 to a discharging signal −VCCthrough a current sink 624 in response to a “Ramp Down” signal.

The exemplary timing circuitry 440 operates in a time generation andmeasurement mode. In an embodiment, the time generation and measurementmode is initialized by setting a counter, such as counter 418 shown inFIG. 4, to zero and discharging the capacitor 616 by opening switches610 and 614 and closing switch 612. Time generation is started byopening switch 612 commanding a transmitter, such as the transmitter 412shown in FIG. 4, to emit a laser pulse and closing switch 610. Withswitch 610 closed, the gcurrent source 622 begins to charge thecapacitor 616. In an exemplary embodiment, the capacitor 616 is a 0.01μF capacitor and the current source 622 is a 5 mA current source. Thus,as a function of time, the charge on the capacitor 616 linearlyincreases or “ramps up” at +0.5 volts per microsecond, which isapproximately 0.001 volts per foot of target range.

Upon detection of a reflected laser pulse, switch 610 is opened to stopcharging the capacitor 616. The flight time is measured by closingswitch 614 and starting the counter 418. With switch 614 closed, thecurrent sink 624 begins to discharge the capacitor 616 while the counter418 accumulates counts. In an exemplary embodiment, the capacitor 616 isa 0.01 μF capacitor, the current sink 624 is a 2.5 μA current sink, andthe counter 418 is a sixteen bit counter that accumulates counts from a500 kHz time base (not shown). Thus, as a function of time, the chargeon the capacitor 616 linearly decreases or “ramps down” at −0.25 voltsper millisecond, which is approximately two counts per foot of targetrange.

When the charge on the discharging capacitor 616 is equal to or lessthan circuit ground 620, the comparator 618 provides a “Ramp_Zero”signal. In response to the Ramp_Zero signal, switch 614 is opened andthe counter 418 is stopped from accumulating any more counts. The valuein the counter 418 comprises an uncalibrated flight time measurement.The uncalibrated flight time measurement is converted into anuncalibrated range measurement using, for example, the method discussedabove in relation to equation (1). At least one of the uncalibratedflight time measurement and uncalibrated range measurement is stored. Inan embodiment, a plurality of uncalibrated range measurements aregenerated before performing analysis on the plurality of uncalibratedrange measurements, such as calibrating the uncalibrated rangemeasurements.

FIGS. 7A and 7B are exemplary graphical representations illustrating atarget range versus a flight time of a laser beam. Referring to FIG. 7A,line 708 has a slope m1 and illustrates a linear relationship between anactual range and an actual flight time. Thus, line 708 represents asituation in which a rangefinder system has no errors in measuring aflight time. The slope m₁ is dependent upon the speed of the laserpulse. In the example discussed in relation to equation (1) above, therange to a target equals about 0.49178 feet per nanosecond multiplied bythe flight time in nanoseconds. Thus, for that example, the slope m1equals 0.49178 feet per nanosecond.

The dashed line 710 corresponds to an uncalibrated range measurementR_(D). If there were no errors in the rangefinder system, the actualflight time would correspond to the intersection of line 708 and thedashed line 710. However, in the presence of timing errors, therelationship between the uncalibrated range measurement R_(D) and line708 is unknown because the actual flight time corresponding to theuncalibrated range measurement R_(D) is uncertain.

Referring to FIG. 7B, line 720 illustrates a linear relationship betweena range to a target and a flight time in the presence of errors causedby inherent delays in internal rangefinder electronics. For example, atransmitter and a receiver contribute a small amount of delay to theround trip flight time measurement. Further, the rise time of thereceiver output signal is a function of the strength of the reflectedlaser pulse signal at the receiver. To calibrate for these propagationdelays, line 720 is shifted by a range calibration value B whilemaintaining the same slope m₁ as line 708 in FIG. 7A.

In one embodiment, the range calibration value B equals a first memoryconstant B1 (not shown) when the reflected laser pulse signal at thereceiver is relatively strong compared to a maximum receiver signal.Similarly, the range calibration value B equals a second memory constantB2 (not shown) when the reflected laser pulse signal at the receiver isrelatively weak compared to the maximum receiver signal.

In one embodiment, the first memory constant B1 and the second memoryconstant B2 are generated during factory calibration and alignment ofthe rangefinder and are stored, for example, in electronicallyaccessible medium, such as a nonvolatile memory within the rangefinder.For example, the user input device 432 or the communication device 436shown in FIG. 4 may be used to store the first memory constant B1 andthe second memory constant B2 in the memory device 420 of therangefinder system 400.

In one embodiment, the range calibration value B is selected from arange of values based upon the received signal strength. In an exemplaryembodiment, the range of values includes a linear relationship betweenthe first memory constant B1 and the second memory constant B2proportional to the received signal strength. Thus, the rangecalibration value B is selected as a function of the received signalstrength.

The dashed line 710 in FIG. 7B again corresponds to an uncalibratedrange measurement R_(D). If propagation delays corresponding to therange calibration value B were the only errors in the timingmeasurement, the actual flight time would correspond to the intersectionof line 720 and the dashed line 710. However, in the presence of timingerrors that vary with time or environmental conditions, the relationshipbetween the uncalibrated range measurement R_(D) and line 720 is unknownbecause the actual flight time corresponding to the uncalibrated rangemeasurement R_(D) is uncertain.

FIGS. 8A and 8B are exemplary graphical representations illustratingcharge in a capacitor during first and second calibration measurements.Referring to FIGS. 6, 8A and 8B, the timing circuitry 440 operates in acalibration mode. In an embodiment, two calibration measurements areperformed. FIG. 8A illustrates the charge on the capacitor 616 as afunction of time during a first calibration measurement and FIG. 8Billustrates the charge on the capacitor 616 as a function of time duringa second calibration measurement. The first calibration measurement isinitialized or “Reset” by disabling the receiver and removing any chargein the capacitor 616 by opening switches 610 and 614 and closing switch612. Calibration time generation is started by opening switch 612transmitting a laser pulse and closing switch 610. With switch 610closed, the current source 622 begins to charge the capacitor 616 asindicated by line 810 in FIG. 8A.

At a predetermined first calibration time T_(A) after transmitting thelaser pulse, switch 610 is opened to stop the charging of the capacitor616. The charge in the capacitor 616 is held until a first simulatedflight time T_(AS) measurement is determined by measuring the timerequired to discharge the capacitor 616. Switch 614 is closed to startdischarging the capacitor 616 through the current sink 624 as indicatedby line 812. When the charge on the discharging capacitor 616 is equalto or less than circuit ground 620, the discharge is complete and thecomparator 618 provides a Ramp_Zero signal. In response to the Ramp_Zerosignal, switch 614 is opened and the first simulated flight time T_(AS)measurement is recorded. As discussed above with respect to equation(1), the first simulated flight time T_(AS) can be converted to a firstsimulated range R_(A) (not shown).

The second calibration measurement is initialized or “Reset” bydisabling the receiver and removing any charge on the capacitor 616 byopening switches 610 and 614 and closing switch 612. Calibration timegeneration is started by opening switch 612, transmitting a laser pulseand closing switch 610. With switch 610 closed, the current source 622begins to charge the capacitor 616 as indicated by line 820 in FIG. 8B.

At a predetermined second calibration time T_(B) after transmitting thelaser pulse, switch 610 is opened to stop the charging of the capacitor616. The charge in the capacitor 616 is held until a second simulatedflight time T_(BS) measurement is determined by measuring the timerequired to discharge the capacitor 616. Switch 614 is closed to startdischarging the capacitor 616 through the current sink 624 as indicatedby line 822. When the charge on the discharging capacitor 616 is equalto or less than circuit ground 620, the discharge is complete and thecomparator 618 provides a Ramp_Zero signal. In response to the Ramp_Zerosignal, switch 614 is opened and the second simulated flight time T_(BS)measurement is recorded. As discussed above with respect to equation(1), the second simulated flight time T_(BS) can be converted to asecond simulated range R_(B) (not shown).

FIGS. 9A and 9B are exemplary graphical representations illustrating atarget range versus a flight time of a laser beam, an uncalibrated rangemeasurement, a first simulated range measurement, and a second simulatedrange measurement. FIG. 9A illustrates line 708 and line 710 of FIG. 7Awith slope m₁ and representing the linear relationship between an actualrange and an actual flight time, and the uncalibrated range measurementR_(D), respectively. FIG. 9A also illustrates a first calibration point910 and a second calibration point 912 defining a calibration line 914having the form of a linear equation:y=mx+b   (2),where y corresponds to the Range axis, m is the slope m₂ of thecalibration line 914, x corresponds to the Flight Time axis, and bcorresponds to the interception of the calibration line 914 with theRange axis at b₁.

The first calibration point 910 corresponds to the first simulated rangeR_(A) and the predetermined first calibration time T_(A). The secondcalibration point 912 corresponds to the second simulated range R_(B)and the predetermined second calibration time T_(B). Thus, the slope m₂of line 914 is defined by:m ₂=(R _(B)−R _(A))/(T _(B)−T _(A))   (3).

Having defined the calibration line 914, uncalibrated data point 916corresponding to uncalibrated range measurement R_(D) is defined by theinterception of line 710 and calibration line 914. The interception ofline 710 and calibration line 914 is found by solving equation (2) for xcorresponding to a calibrated flight time T_(C). Thus, the calibratedflight time T_(C) is given by:T _(C)=(R _(D)−b ₁)/m ₂   (4).

Once the calibrated flight time T_(C) is known, the uncalibrated datapoint 916 is correlated to a calibrated data point 920 along line 708 atthe calibrated flight time T_(C). The calibrated data point 920corresponds to a calibrated range R_(C) which is determined by solvingequation (2) where y is the calibrated range R_(C), m is slope m1, x isthe calibrated flight time T_(C) defined by equation (4), and b is zero.Thus, the calibrated range R_(C) is given by:R _(C)=m ₁ (T _(C))+0=m ₁ (R _(D)−b ₁)/m₂   (5).

In the example discussed in relation to equation (1) above, the range toa target equals 0.49178 feet per nanosecond multiplied by the flighttime in nanoseconds. Thus, for that example, the slope m₁ equals 0.49178feet per nanosecond. Substituting this value for m₁ and equation (3) form₂ in equation (5) gives:R_(C)=(R _(D)−b ₁) (0.49178/m ₂)=(R _(D)−b ₁) (0.49178/(R _(B)−R _(A)))(T _(B)−T _(A))   (6).

FIG. 9B illustrates line 720 and line 710 of FIG. 7B in relation to thefirst calibration point 910 and the second calibration point 912 shownin FIG. 9A. As discussed above, line 720 illustrates a linearrelationship between a range to a target and a flight time in thepresence of errors caused by delays in internal rangefinder electronics.Line 720 is shifted by a range calibration value B while maintainingslope m₁. The calibration line 914, slope m₂, uncalibrated data point916, and calibrated flight time are each determined as described above.

In FIG. 9B, the uncalibrated data point 916 is correlated to acalibrated data point 922 along line 720 at the calibrated flight timeT_(C). The calibrated data point 922 corresponds to a calibrated rangeR_(C)′ which is determined by solving equation (2) where y is thecalibrated range R_(C)′, m is slope m₁, x is the calibrated flight timeT_(C) defined by equation (4), and b is B. Thus, the calibrated rangeR_(C)′ is given by:R _(C)′=(m ₁(T _(C)))+B=( m ₁(R _(D)−b ₁)/m₂)+B   (7).In the example discussed in relation to equation (1) above, the slope m₁equals 0.49178 feet per nanosecond. Substituting this value for m₁ andequation (3) for m₂ in equation (7) gives:R _(C)′=((R _(D)−b ₁) (0.49178/(R _(B)−R _(A))) (T _(B)−T _(A)))+B  (8).

Therefore, by generating at least two simulated calibration measurementsfor each range measurement or group of range measurements, time-varyingrange measurement errors can be corrected by using equation (6).Further, time-varying range measurement errors and errors due to atransmitter, receiver and receiver signal strength can be corrected byusing equation (8).

Although the present invention has been described with reference tospecific embodiments, other embodiments will occur to those skilled inthe art. For example, timing circuitry, such as the timing circuitry 440shown in FIG. 4, may comprise a high speed counter (not shown) driven bya time base (not shown) such as an oscillator. In an embodiment, thehigh speed counter is configured to accumulate counts from the time basewhen a beam is emitted and to stop accumulating counts from the timebase when a beam is detected. Thus, the flight time of an emitted andreflected beam is proportional to the counts accumulated by the highspeed counter. In such an embodiment, calibration is provided byaccounting for inherent delays in internal rangefinder electronics suchas discrete component delays in the transmitter and receiver as well asdelays caused by the rise time of the receiver output signal as afunction of the strength of the received signal.

For example, taking the distance traveled by the transmitted andreflected beams at an exemplary speed to be about 6.1002 nanoseconds peryard and the high speed counter to accumulate counts at an exemplarycount of about 6.25 nanoseconds per count, the range to the target maybe calculated as:Range(yards)=(((6.25/6.1002)×raw range count)+B)   (9),where 6.25 nanoseconds per count and 6.1002 nanoseconds per yard areprovided for exemplary reasons only and are not intended to limit orconstrue the disclosure or claims. The “raw range count” is the countaccumulated by the high speed counter while measuring the flight time ofthe transmitted and reflected beams.

The “B” term in equation (9) represents a range calibration value. In anembodiment, the range calibration value B equals a first memory constantwhen the reflected beam signal is relatively strong compared to amaximum receiver signal. Similarly, the range calibration value B equalsa second memory constant when the reflected beam signal is relativelyweak compared to the maximum receiver signal. In an embodiment, therange calibration value B is selected from a range of values between thefirst memory constant and the second memory constant so as to beproportional to the strength of the reflected beam signal. In anembodiment, the first memory constant and the second memory constant aregenerated during factory calibration and alignment of the rangefinderand are stored, for example, in electronically accessible medium, suchas a nonvolatile memory within the rangefinder.

It is to be understood that the embodiments described above have beenpresented by way of example, and not limitation, and that the inventionis defined by the appended claims.

1. A method for estimating a range to a target using a rangefinder, therangefinder comprising a transmitter and a receiver, the methodcomprising: emitting a beam from the transmitter towards the target;detecting a reflected beam from the target in the receiver, wherein aflight time comprises a time between emitting the beam and the detectingthe reflected beam; accumulating counts at least during the flight time;and determining the range to the target from the counts.
 2. The methodof claim 1, wherein the counts are accumulated in a high speed counter3. The method of claim 1, wherein accumulating the counts includesgenerating the counts from a time base.
 4. The method of claim 1,further comprising: calibrating the rangefinder, the calibrationincluding: determining a first relationship between said flight time andsaid range, the first relationship including a first slope; generating afirst simulated range measurement for a first calibration flight time;generating a second simulated range measurement for a second calibrationflight time; determining a second relationship between said flight timeand said range based on the first and second calibration flight timesand the first and second simulated range measurements, the secondrelationship including a second slope; correlating the counts to thefirst relationship using with the second relationship; and determiningthe range to the target using the correlation.
 5. The method of claim 4,wherein correlating the counts to the first relationship comprises:correlating the counts to the second relationship; determining acalibrated time; and correlating the calibrated time to the firstrelationship to determine a calibrated range.
 6. The method of claim 4,wherein the first slope is proportional to a speed of the beam.
 7. Themethod of claim 4, wherein the second slope is proportional to a ratioof a difference between the first and second simulated rangemeasurements and a difference between the first and second calibrationflight times.
 8. The method of claim 4, further comprising: selecting arange calibration value; and shifting the first relationship by therange calibration value.
 9. The method of claim 8, wherein selecting therange calibration value comprises measuring a strength of a detectedbeam.
 10. The method of claim 8, wherein selecting the range calibrationvalue comprises choosing the range calibration value from a range ofvalues stored in a memory.
 11. A method for estimating a distance to atarget using a rangefinder, the rangefinder comprising an emitter and areceiver, the method comprising: emitting a beam from the emittertowards the target; detecting a reflected beam from the target in thereceiver; at least between the emitting and the detecting, acquiringprocessor data representative of time; and determining the distance tothe target using the processor data.
 12. A method for estimating a rangeto a target, the method comprising: emitting a beam towards a target;starting a timer; detecting a reflected beam from the target; stoppingthe timer; and determining the range to the target using data acquiredfrom the timer, wherein a flight time comprises time between at leastthe emitting and the detecting, wherein the timer runs at least duringthe flight time, and wherein the timer comprises a processor.
 13. Arangefinder configured to determine a range to a target, the rangefindercomprising: a transmitter configured to emit a beam towards the target;a receiver configured to detect a reflected beam from the target; andtiming circuitry including a processor, the timing circuitry configuredto measure a flight time between at least the emission of the beam fromthe transmitter and the detection of the reflected beam by the receiver.14. The rangefinder of claim 13, wherein the flight time is proportionalto the counts.
 15. The rangefinder of claim 13, comprising anoscillator.
 16. The rangefinder of claim 13, wherein the transmittercomprises a laser diode configured to emit a pulsed laser beam.
 17. Therangefinder of claim 13, further comprising: a calibration sectionconfigured to determine calibration data related to dynamic factors; anda processor configured to adjust the flight time based on thecalibration data.
 18. The rangefinder of claim 17, wherein thecalibration section is further configured to simulate first and secondrange measurements.
 19. The rangefinder of claim 18, wherein theprocessor is further configured to correlate the flight time to thefirst and second simulated range measurements.
 20. The rangefinder ofclaim 17, wherein the dynamic factors comprise range measurement errorsthat vary with environmental conditions.
 21. The rangefinder of claim17, wherein the processor is further configured to adjust the flighttime based on inherent delays of the rangefinder.
 22. The rangefinder ofclaim 17, wherein the processor is further configured to adjust theflight time according to a strength of a detected beam.
 23. Therangefinder of claim 17, further comprising a memory, the memoryincluding at least two range calibration values.
 24. The rangefmder ofclaim 23, wherein the processor is further configured to adjust theflight time based on a calibration value between the at least two rangecalibration values.
 25. A system for measuring a range to a target, thesystem comprising: a means for transmitting a beam towards a target; ameans for detecting a beam reflected from the target; and a means formeasuring data during the flight time, the flight time comprising timeat least between transmission of the beam and detection of the reflectedbeam.
 26. The system of claim 25, wherein the measuring means comprisesa high speed counter driven by a time base and wherein the datacomprises counts.
 27. The system of claim 26, wherein the time basecomprises an oscillator.